Vincent Gable’s Blog

This bug was frustrating enough that half-way through squashing it, I promised myself I would document the solution. Unfortunately, it’s not a particularly interesting bug, but a promise is a promise!

I had some code very much like:

typedef enum {NoError, ReadError, WriteError, NetworkError, UnknownError} ErrorType;
...
void DescriptionOfError(ErrorType err, char *string)
{
   switch(err)
   {
      IOError:
         strcpy(string, "Could not read data.");
         break;
      
      WriteError:
         strcpy(string, "Could not write data!");
         break;

int main (int argc, char** argv)
{
   char desc[1024];
   DescriptionOfError(ReadError,desc);
   printf("error: %s\n", desc);
   return 0;
}


And it just wouldn’t do the right thing, but for the life of me I couldn’t see what was wrong.

The “solution” was very simple, and somewhat embarrassing. I forgot the case keyword before the labels in the switch statement. Turns out that if you don’t have a case in-front of a label, it’s treated like a goto label, not a switch label. And this is something that I’ve known for years, but for 20 minutes I kept reading the code, and my brain would interpret what it should say, not literally analyze it.

This was extremely frustrating, not because it took me a long time to fix (I wish all my bugs could be squashed in just 20 minutes!), but because the results I was seeing totally violated my mental model of what should have happened. Violating someone’s mental model is more unsettling then you might imagine — avoid doing it at all costs.